The present invention relates generally to tracking of induction machines without rotor position or speed sensors.
The elimination of rotor shaft transducers is highly desirable to reduce cost, reduce total motor package size, and to improve system reliability. Rotor shaft transducers tend to be a major source of failure and expense in ac motor drives in both industrial and traction applications.
Prior approaches to tracking and control of induction machines without rotor position or speed sensors are described, for example, in Jansen et al., U.S. Pat. Nos. 5,565,752 and 5,585,709, and in Cilia et al., "Sensorless Position Detection for Vector-Controlled Induction Motor Drives Using an Asymmetric Outer-Section Cage," IEEE Transactions on Industry Applications, Vol. 33, No. 5, September/October 1997 pp. 1162-69. These embodiments have difficulty in robustly tracking rotor resistance variations in motors with closed-rotor slots because saturation of the rotor slot bridges creates a variable spatial modulation or asymmetry in the rotor leakage inductance that corrupts the tracking methods. Most induction machines are fabricated with aluminum die-cast rotor cages. In these motors, closed-rotor-slot designs are used to contain the molten aluminum during casting and are thus the predominate slot type to minimize manufacturing costs.